An electroless nickel plating solution has hitherto been generally composed of nickel sulfate as a nickel source in combination with sodium hypophosphite as a reducing agent. Sodium hydroxide or ammonium hydroxide has also been used to maintain the resulting plating solution at a predetermined pH level. When nickel is electrolessly plated using a plating solution of such composition, sodium sulfate is formed and, over time, accumulates in the plating solution. The sodium sulfate results from a reaction between a sodium phosphite derived from an oxidation of the sodium hypophosphite and a sulfate group derived from the nickel sulfate for use as the nickel source. This accumulation causes the plating solution to age, eventually inviting phenomena such as reduced plating speed, extraordinary deposition, deteriorated film quality and the like. Thus, after being used for a certain period of time the plating solution is replenished, for continuous use, with fresh liquid chemicals to thereby compensate for those components that have been used up. When in a state wherein both the sodium sulfate and the sodium phosphite are contained in high concentrations, the plating solution is not recycled and ultimately disposed in the ocean or the like as a waste liquid.
From the point of view of global environmental protection, however, the above mentioned plating solution waste has been banned from oceanic disposal since 1995 by the provisions of the London Dumping Treaty. Consequently, a demand has been voiced for the development of an electroless plating solution that affords prolonged service life or permits disposal with minimal environmental pollution.
Various attempts have been made to remove sodium phosphite and sodium sulfate which are built up during electroless nickel plating as discussed hereinbefore, but none of them are being practiced on a commercial basis. The methods which have been proposed for the removal of the sodium phosphite and sodium sulfate materials include, for instance, a method in which an electrolysis membrane is employed, a method in which an aluminum sheet or an aluminum foil is placed in an aged alkaline plating solution in such a method that nickel is deposited, followed by recovering the resultant nickel with nitric acid and then allowing a chelate resin to adsorb a portion of the remaining undeposited nickel in the mother liquor (Japanese Patent Laid-Open No. 51-6136); and a method in which nickel ions and sodium ions are separated from a plating solution in advance by an ion exchange resin such that sodium and nickel are separated from each other by adsorption into the ion exchange resin, the nickel alone being put back into the plating solution for subsequent reuse.
Still another method has been adopted in which part of an aged plating solution is disposed of and replaced with fresh liquid chemicals so as to extend the life time of the plating solution.
These, however, though likely to be regarded as rational processes in some respects, have too many economic and technological drawbacks to be suitable for practical application. Moreover, the method of partially substituting fresh liquid chemicals for an aged plating solution cannot be said to offer a fundamental solution to the problem.
Accordingly, the object of the present invention is to provide an industrially advantageous method which enables reuse of an aged electroless nickel plating solution. More particularly, the invention provides a method for circulating an electroless nickel plating solution which is as non-polluting as possible to the global environment.